This lecture introduces the fundamentals of biomedical imaging, focusing on the definition of bio-imaging as the localized measurement of contrast generating biophysical effects in living organisms. It covers the concept of contrast, the difference between signal and noise, and the importance of contrast-to-noise ratio. The difference between signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) is explained, emphasizing the need for good measurements in imaging. Examples of biomedical images, such as X-ray diffraction of DNA crystals and blood cells, are presented to illustrate the concepts discussed.